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Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 26 — Dec. 25, 2006
  • pp: 12814–12821
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Ultra-compact microdisk resonator filters on SOI substrate

Alain Morand, Yang Zhang, Bruno Martin, Kien Phan Huy, David Amans, Pierre Benech, Jérémy Verbert, Emmanuel Hadji, and Jean-Marc Fédéli  »View Author Affiliations


Optics Express, Vol. 14, Issue 26, pp. 12814-12821 (2006)
http://dx.doi.org/10.1364/OE.14.012814


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Abstract

The evanescent coupling of a 1.5 µm radius silicon microdisk with one or two Silicon-On-Insulator waveguides is studied. Thanks to the high refractive index contrast between Silica and Silicon materials, this very-small-diameter microdisk exhibits the highest quality factor measured in wavelength range from 1500 nm to 1600 nm. Coupled to a single monomode waveguide, the optical resonator behaves as a stop-band filter. Even if the microdisk is a largely multimode resonator, only its fundamental modes are efficiently excited. The filter’s transmission is measured for different gap between the waveguide and the resonator. The critical coupling is clearly observed and gives access to 1.63 nm linewidth. A 20 dB decrease of the transmission signal is also observed. Coupled to two waveguides, the resonator becomes a compact symmetric wavelength-demultiplexer. In this case, the optimal response comes from a compromise between the gap and the desired linewidth dropped in the second waveguide. Finally, our measurements are also compared to analytic models showing a good agreement especially for the critical gap prediction.

© 2006 Optical Society of America

1. Introduction

The increase density of metallic-interconnects becomes a real barrier for future generation of Very-Large-Scale-Integrated circuits. Optical interconnects using Silicon-On-Insulator (SOI) technology decrease the propagation delays and give access to higher bandwidth [1

1. M. J. Kobrinsky, B. A. Block, J.-F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Robertson, S. List, I. Young, and K. Cadien, “On-Chip Optical Interconnects,” Intel Technol. J. 8, 129–142 (2004).

, 2

2. G. Tosik, F. Gaffiot, Z. Lisik, I. O’Connor, and F. Tissafi-Drissi, “Power dissipation in optical and metallic clock distribution networks in new VLSI technologies,” Electron. Lett. 40, 198–200 (2004). [CrossRef]

]. In order to perform the on-chip information routing, wavelength resonator filter based on cylindrical geometries are investigated for add and drop [3

3. B. E. Little, S. T. Chu, H. A. Haus, J. Foresi, and J-P. Laine, “Microring resonator channel dropping filters,” J. Lightwave Technol. 15, 998–1005 (1997). [CrossRef]

, 4

4. W. Bogaerts, D. Taillaert, B. Luyssaert, P. Dumon, J. Van Campenhout, P. Bienstman, D. Van Thourbout, R. Baets, V. Wiaux, and S. Beckx, “Basic structures for photonic integrated circuits in silicon-on-insulator,” Opt. Express 12, 1583–1591 (2004). [CrossRef] [PubMed]

] or stop band filter [5

5. P. P. Absil, J. V. Hryniewicz, B. E. Little, R. A. Wilson, L. G. Joneckis, and P. T. Ho, “Compact microring notch filters,” IEEE Photon. Technol. Lett. 12, 398–400 (2000). [CrossRef]

] applications. Thanks to high refractive-index contrast between silica and silicon materials, ultra-compact structures can be made without using peculiar mirrors. Recently, high-resonant microring cavities [6

6. J. Niehusmann, A. Vorckel, and P. Aring Bolivar, “Ultrahigh-quality-factor silicon-on-insulator microring resonator,” Opt. Lett. 29, 2861–2863 (2004). [CrossRef]

] have been achieved with diameters close to 20 µm. The quality factor obtained gives access to several applications on SOI substrate such as high-speed modulator [7

7. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435, 325–327 (2005). [CrossRef] [PubMed]

] or compact Raman sources [8

8. Q. Xu, V. R. Almeida, and M. Lipson, “Demonstration of high Raman gain in a submicrometer size silicon-on-insulator waveguide,” Opt. Lett. 30, 35–37 (2005). [CrossRef] [PubMed]

]. Compared to other ultra-resonant cavities as microtorus [9

9. T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83, 797–799 (2003). [CrossRef]

] or microsphere [10

10. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralow-threshold raman laser using a spherical dielectric microcavity,” Nature 415, 621–623 (2002). [CrossRef] [PubMed]

], microring has the great advantage to be integrated on a substrate and to be easily associated to waveguide couplers. Nevertheless, the resonator size can be reduced using microdisk instead of microring [11

11. E. A. J. Marcatili, “Bends in optical dielectric guides,” Bell Syst. Tech. J. 48, 2103–2132 (1969).

]. Indeed, the deep etch for designing the inner radius induces a displacement of the field to the external radius. By consequence, the field is less confined in the resonator. It is also more sensible to the roughness on each boundaries of the bend waveguide reducing the quality factor. In this paper, two basic optical-interconnects functions using microdisks are reported. The former is a stop-band filter obtained from a microdisk coupled to a single waveguide. The amplitude transmission of the waveguide is then described by the following relation [12

12. V. S. Ilchenko and A. B. Matsko, “Optical resonators with whispering gallery modes part II: applications,” IEEE J. Sel. Top. Quantum Electron. 12, 15–32 (2006). [CrossRef]

]

T=γcγ+i(ωω0)γc+γ+i(ωω0),
(1)

T=γcγc+i(ωω0),
(2)
R=i(ωω0)γc+i(ωω0),
(3)

2. Realization process and set up characterisation

A 193 nm deep UV lithography is used to reach small propagation losses and a gap close to one hundred nanometers. All the waveguiding structures are realized on a SiO2 layer of 1 µm thickness and are overlayed by a silica box of 1 µm thickness as shown in Fig. 1(a). The thickness of the Silicon structures is set to 300 nm. To obtain monomode waveguides along the wavelength range from 1.5 µm to 1.6 µm, a width of 300 nm was defined on the mask. The measurements are performed only for the transverse magnetic field. As a consequence, a polarization-maintaining fiber is used to inject the TM polarized light from a polarized laser source. To couple the light in and out of these small waveguides, horizontal tapers are put at the input and output of the waveguide. They enlarge the width of the waveguide from 0.3 µm to 2 µm. A SEM picture of the add-and-drop filter is shown in Fig. 1(b). Several small microdisks with diameter of 3 µm have been thus made. In order to characterize the different structures, two three-dimensional (3-D) translation positioners are used to set a polarization-maintaining tapered fiber and a standard tapered fiber at the input and output of the waveguides respectively. A tunable laser source is used to scan the wavelength range from 1.48 µm to 1.62 µm. The output fiber is connected to a photo-detector. The wavelength scan and the power acquisition are controlled by a soft process.

Fig. 1. (a). Description of the different layers used in a SOI photonic component. (b) SEM picture of a symmetric add-and-drop filter before the overlaying box is grown.

3. Stop band filter

The characterization of the stop-band filter is presented in this part. Examples of power transmission |T|2 are shown in the Fig. 2. To understand those spectra, we shall point out the fact that the microdisk is a multimode structure [14

14. K. P.. Huy, J. Verbert, F. Mazen, P. Noé, J. M. Gérard, E. Hadji, F. Orucevic, J. Hare, V. Lefèvre-Seguin, A. Morand, and P. Benech, “Room temperature of Er-doped silicon-rich oxide microcavities supporting high-Q whispering gallery modes,” in Nanophotonic Materials and Systems II: Silicon nanophotonics, Z. Gaburro and S. Cabrini, eds., Proc. SPIE592559250O (2005). [CrossRef]

]. A WGM is defined by an azimuthal order m (number of period of the field along the periphery of the microdisk) and a radial order l (number of period in the radial direction in the microdisk). In the transmission response, only two resonance peaks are observed in the chosen wavelength range. These two WGMs have the same radial order l=0 and two different azimuthal orders respectively m and m+1 and a free spectral range of 67.4 nm is measured. Although there is other resonant modes, only the most resonant modes (l=0) are excited [15

15. A. Morand, K. Phan-Huy, P. Benech, and Y. Désières, “Analytical study of the microdisk coupling with a waveguide based on the perturbation theory,” J. Lightwave Technol. 22, 827–832 (2004). [CrossRef]

] that can be correlated by smallest efficiency of their coupling linewidth. Higher radial-order modes are not efficiently excited since their overlap with the waveguide mode seems to be very poor. In the pass-band wavelength range, the level of the transmission is not constant. With a higher wavelength resolution (Δλ=1 pm), an oscillation is clearly observed. This oscillation comes from the Fabry-Perot resonance induced by Fresnel reflexions at the input and output interfaces of the waveguide.

Fig. 2. Power transmission in a stop band filter with a gap of 230 nm.(a) λres=1524.73 nm with m=13. (b) λres=1592.37 nm with m=12.

To study the influence of the gap, the mask included gaps going from 150 nm to 310 nm for the same disk diameter. To obtain the value of the resonant wavelength, the quality-factor and the extinction ratio of the transmission, each amplitude transmission responses is fitted by a Lorentzian lineshape. The results are depicted in Fig. 3(a), Fig. 3(b), and Fig. 3(c).

Fig. 3. (a). Evolution of the wavelength resonance with the gap for the two azimuthal orders m and m+1. (b) Evolution of the quality factor with the gap for the two azimuthal orders m and m+1. (c) Evolution of the extinction ratio with the gap for the two azimuthal orders m and m+1.

Table 1. Comparison between the measurements and calculated values obtained with the analytical method.

table-icon
View This Table

Calculated and measured resonant wavelengths are very close. Note that the stronger mismatch for the quality-factor can be explained by the small number of gaps used in the measurements and the rapid variation of the quality-factor around the critical point. A weak roughness of the microdisk sidewalls explains the good quality-factors measured with this small cavity.

4. Symmetric wavelength demultiplexer

In this paragraph, the characterization of the symmetric add-and-drop is finally presented. For a gap equal to 230 nm, two resonance peaks are obtained in our wavelength range shown in Fig. 4(a). On the T (transmission) response only two peaks are also observed which confirm the efficient coupling of only two resonances even if the microdisk is a largely multimode resonator. Similarly to section 1, the resonant peaks are fitted by a Lorentzian lineshape to reach the different optical parameters as shown in the Fig. 4(b). The obtained resonant wavelength and the quality-factor are depicted in Fig. 5(a) and Fig. 5(b) showing similar behaviour to the stop-band filter of section 3.

Fig. 4. (a). Power transmission from 1500 nm to 1600 nm with a gap of 230 nm (b) Power transmission of the first resonance peak.
Fig. 5. (a). Evolution of the wavelength resonance with the gap for the azimuthal order m=13. (b) Evolution of the quality factor with the gap for the azimuthal order m=13. (c) Evolution of the extinction ratio on the R reflection with the gap for the azimuthal order m=13.

As expected, no critical point is reached. The maximum value is obtained for a small gap but in this case the quality-factor is very low and induces a large pass bandwith as shown in Fig. 5(b). If a quality factor of 1000 is necessary, the extinction ratio will be slightly 8 dB for a gap around 270 nm. In comparison with the stop band filter results, one may see that the second waveguide decreases the quality-factor for small gaps. Indeed the second coupling waveguide increases the optical light extracted from the microdisk. To obtain a better extinction ratio the gaps of the two waveguides must be different. In this case, due to the intrinsic losses of the single microdisk, the efficiency of the structure is optimized only in one way (demultiplexer or multiplexer functions) [17

17. A. Vörckel, M. Mönster, W. Henschel, P. H. Bolivar, and H. Kurz, “Asymmetrically coupled silicon-on-insulator microring resonators for compact add-drop multiplexers,” IEEE Photon. Technol. Lett. 15, 921–923 (2003). [CrossRef]

]. We also measured other add-and-drop with a larger diameter. And for a 3.2 µm diameter instead of 3 µm, extinction ratio of 22 dB with a quality factor of 918 has been measured for a gap of 220 nm. A weak increase of the diameter allows us to reach higher quality factor with the same efficiency for the minimum of transmission in the input waveguide [18

18. A. Morand, K. P. Huy, B. Martin, F. Bredillot, D. Amans, P. Benech, J. Verbert, E. Hadji, and J-M. Fedeli, “Compact add-and-drop and wavelength filter based on microdisk on SOI substrate,” in Silicon Photonics, J. A. Kubby and G. T. Reed, eds., Proc. SPIE 6125, 192–199 (2006).

].

5. Conclusion

In this paper, ultra-compact resonant SOI structures based on 3 µm diameter microdisk coupled to one or two waveguides have been presented. The gap between the microdisk of the waveguides has been especially studied. For the stop-band filter, the critical point has been demonstrated and an extinction ratio of 20 dB is obtained with a quality factor around 1000. The comparison with an analytical method based on the effective index method and the coupled-mode-theory shows a remarkable fit. For the symmetric add-and-drop structure, the exctinction ratio is around 8 dB if the quality factor is at the same order as previously. Finally, it is clearly confirmed that even if the microdisk is a multimode structure only the desired mode are only excited thanks to the poor overlap with the higher radial-order modes.

Acknowledgments

This work was partially supported by the ACI Program from the French ministry of research and education.

References and links

1.

M. J. Kobrinsky, B. A. Block, J.-F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Robertson, S. List, I. Young, and K. Cadien, “On-Chip Optical Interconnects,” Intel Technol. J. 8, 129–142 (2004).

2.

G. Tosik, F. Gaffiot, Z. Lisik, I. O’Connor, and F. Tissafi-Drissi, “Power dissipation in optical and metallic clock distribution networks in new VLSI technologies,” Electron. Lett. 40, 198–200 (2004). [CrossRef]

3.

B. E. Little, S. T. Chu, H. A. Haus, J. Foresi, and J-P. Laine, “Microring resonator channel dropping filters,” J. Lightwave Technol. 15, 998–1005 (1997). [CrossRef]

4.

W. Bogaerts, D. Taillaert, B. Luyssaert, P. Dumon, J. Van Campenhout, P. Bienstman, D. Van Thourbout, R. Baets, V. Wiaux, and S. Beckx, “Basic structures for photonic integrated circuits in silicon-on-insulator,” Opt. Express 12, 1583–1591 (2004). [CrossRef] [PubMed]

5.

P. P. Absil, J. V. Hryniewicz, B. E. Little, R. A. Wilson, L. G. Joneckis, and P. T. Ho, “Compact microring notch filters,” IEEE Photon. Technol. Lett. 12, 398–400 (2000). [CrossRef]

6.

J. Niehusmann, A. Vorckel, and P. Aring Bolivar, “Ultrahigh-quality-factor silicon-on-insulator microring resonator,” Opt. Lett. 29, 2861–2863 (2004). [CrossRef]

7.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435, 325–327 (2005). [CrossRef] [PubMed]

8.

Q. Xu, V. R. Almeida, and M. Lipson, “Demonstration of high Raman gain in a submicrometer size silicon-on-insulator waveguide,” Opt. Lett. 30, 35–37 (2005). [CrossRef] [PubMed]

9.

T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83, 797–799 (2003). [CrossRef]

10.

S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultralow-threshold raman laser using a spherical dielectric microcavity,” Nature 415, 621–623 (2002). [CrossRef] [PubMed]

11.

E. A. J. Marcatili, “Bends in optical dielectric guides,” Bell Syst. Tech. J. 48, 2103–2132 (1969).

12.

V. S. Ilchenko and A. B. Matsko, “Optical resonators with whispering gallery modes part II: applications,” IEEE J. Sel. Top. Quantum Electron. 12, 15–32 (2006). [CrossRef]

13.

A. Yariv,“Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36, 321–322 (2000). [CrossRef]

14.

K. P.. Huy, J. Verbert, F. Mazen, P. Noé, J. M. Gérard, E. Hadji, F. Orucevic, J. Hare, V. Lefèvre-Seguin, A. Morand, and P. Benech, “Room temperature of Er-doped silicon-rich oxide microcavities supporting high-Q whispering gallery modes,” in Nanophotonic Materials and Systems II: Silicon nanophotonics, Z. Gaburro and S. Cabrini, eds., Proc. SPIE592559250O (2005). [CrossRef]

15.

A. Morand, K. Phan-Huy, P. Benech, and Y. Désières, “Analytical study of the microdisk coupling with a waveguide based on the perturbation theory,” J. Lightwave Technol. 22, 827–832 (2004). [CrossRef]

16.

S. V. Boriskina, T. M. Benson, P. Sewell, and A. I. Nosich, “Effect of a layered environment on the complex natural frequencies of two dimensional WGM dielectric ring resonators,” J. Lightwave Technol. 20, 1563–1572 (2002). [CrossRef]

17.

A. Vörckel, M. Mönster, W. Henschel, P. H. Bolivar, and H. Kurz, “Asymmetrically coupled silicon-on-insulator microring resonators for compact add-drop multiplexers,” IEEE Photon. Technol. Lett. 15, 921–923 (2003). [CrossRef]

18.

A. Morand, K. P. Huy, B. Martin, F. Bredillot, D. Amans, P. Benech, J. Verbert, E. Hadji, and J-M. Fedeli, “Compact add-and-drop and wavelength filter based on microdisk on SOI substrate,” in Silicon Photonics, J. A. Kubby and G. T. Reed, eds., Proc. SPIE 6125, 192–199 (2006).

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices

ToC Category:
Integrated Optics

History
Original Manuscript: June 22, 2006
Revised Manuscript: August 11, 2006
Manuscript Accepted: August 14, 2006
Published: December 22, 2006

Citation
Alain Morand, Yang Zhang, Bruno Martin, Kien Phan Huy, David Amans, Pierre Benech, Jérémy Verbert, Emmanuel Hadji, and Jean-Marc Fédéli, "Ultra-compact microdisk resonator filters on SOI substrate," Opt. Express 14, 12814-12821 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-26-12814


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References

  1. M. J. Kobrinsky, B. A. Block, J.-F. Zheng, B. C. Barnett, E. Mohammed, M. Reshotko, F. Robertson, S. List, I. Young, K. Cadien, "On-Chip Optical Interconnects," Intel Technol. J. 8, 129-142 (2004).
  2. G. Tosik, F. Gaffiot, Z. Lisik, I. O'Connor, and F. Tissafi-Drissi, "Power dissipation in optical and metallic clock distribution networks in new VLSI technologies," Electron. Lett. 40, 198-200 (2004). [CrossRef]
  3. B. E. Little, S. T. Chu, H. A. Haus, J. Foresi and J-P. Laine, "Microring resonator channel dropping filters," J. Lightwave Technol. 15, 998-1005 (1997). [CrossRef]
  4. W. Bogaerts, D. Taillaert, B. Luyssaert, P. Dumon, J. Van Campenhout, P. Bienstman, D. Van Thourbout, R. Baets, V. Wiaux and S. Beckx, "Basic structures for photonic integrated circuits in silicon-on-insulator," Opt. Express 12, 1583-1591 (2004). [CrossRef] [PubMed]
  5. P. P. Absil, J. V. Hryniewicz, B. E. Little, R. A. Wilson, L. G. Joneckis and P. T. Ho, "Compact microring notch filters," IEEE Photon. Technol. Lett. 12, 398-400 (2000). [CrossRef]
  6. J. Niehusmann, A. Vorckel, and P. Aring Bolivar, "Ultrahigh-quality-factor silicon-on-insulator microring resonator," Opt. Lett. 29, 2861-2863 (2004). [CrossRef]
  7. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, "Micrometre-scale silicon electro-optic modulator," Nature 435, 325-327 (2005). [CrossRef] [PubMed]
  8. Q. Xu, V. R. Almeida and M. Lipson, "Demonstration of high Raman gain in a submicrometer size silicon-on-insulator waveguide," Opt. Lett. 30, 35-37 (2005). [CrossRef] [PubMed]
  9. T. J. Kippenberg, S. M. Spillane, D. K. Armani and K. J. Vahala, "Fabrication and coupling to planar high-Q silica disk microcavities," Appl. Phys. Lett. 83, 797-799 (2003). [CrossRef]
  10. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, "Ultralow-threshold raman laser using a spherical dielectric microcavity," Nature 415, 621-623 (2002). [CrossRef] [PubMed]
  11. E. A. J. Marcatili, "Bends in optical dielectric guides," Bell Syst. Tech. J. 48, 2103-2132 (1969).
  12. V. S. Ilchenko and A. B. Matsko, "Optical resonators with whispering gallery modes part II: applications," IEEE J. Sel. Top. Quantum Electron. 12, 15-32 (2006). [CrossRef]
  13. A. Yariv,"Universal relations for coupling of optical power between microresonators and dielectric waveguides," Electron. Lett. 36,321-322 (2000). [CrossRef]
  14. K. P. Huy, J. Verbert, F. Mazen, P. Noé, J. M. Gérard, E. Hadji, F. Orucevic, J. Hare, V. Lefèvre-Seguin, A. Morand and P. Benech, "Room temperature of Er-doped silicon-rich oxide microcavities supporting high-Q whispering gallery modes," in Nanophotonic Materials and Systems II: Silicon nanophotonics, Z. Gaburro, S. Cabrini, eds., Proc. SPIE 5925 59250O (2005). [CrossRef]
  15. A. Morand, K. Phan-Huy, P. Benech and Y. Désières, "Analytical study of the microdisk coupling with a waveguide based on the perturbation theory," J. Lightwave Technol. 22, 827-832 (2004). [CrossRef]
  16. S. V. Boriskina, T. M. Benson, P. Sewell and A. I. Nosich, "Effect of a layered environment on the complex natural frequencies of two dimensional WGM dielectric ring resonators," J. Lightwave Technol. 20, 1563-1572 (2002). [CrossRef]
  17. A. Vörckel, M. Mönster, W. Henschel, P. H. Bolivar and H. Kurz, "Asymmetrically coupled silicon-on-insulator microring resonators for compact add-drop multiplexers," IEEE Photon. Technol. Lett. 15, 921-923 (2003). [CrossRef]
  18. A. Morand, K. P. Huy, B. Martin, F. Bredillot, D. Amans, P. Benech, J. Verbert, E. Hadji, J-M. Fedeli, "Compact add-and-drop and wavelength filter based on microdisk on SOI substrate," in Silicon Photonics, J. A. Kubby, G. T. Reed, eds., Proc. SPIE 6125, 192-199 (2006).

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